Wireless RF receivers are used in a wide variety of applications such as broadcast radio and television, smart metering, remote control, and the like. As used herein, a “radio frequency” signal means an electrical signal conveying useful information and having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc.
An RF receiver generally receives an RF input signal from an antenna or other signal source and adjusts its gain for further processing. In a typical RF receiver front end, a signal from an antenna is input to an attenuator and the attenuator output signal is input to a low noise amplifier (LNA). The attenuator can either pass the signal through unattenuated or apply an attenuation. The attenuator provides proper input impedance to match the source impedance of the antenna for optimum power transfer while also providing appropriate output impedance as seen from the input of the LNA. The LNA then typically converts its single-ended input signal to a differential output signal in order to enable downstream stages to process the signal with better noise and distortion performance.
In one typical LNA configuration, two amplifiers are used to convert a single-ended input signal to a differential output signal; one amplifier generates the positive output and the other generates the negative output. In this configuration, one amplifier has relatively low input impedance to match the output impedance of the attenuator, while the other amplifier has relatively high input impedance but can be used to cancel the noise and distortion introduced by the first amplifier by making the noise and distortion common-mode at the output of the LNA. However in this configuration, the noise and distortion introduced by the second amplifier itself are not cancelled, which limits the overall performance of the LNA.
In the following description, the use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.